The invention concerns an internal combustion engine. More particularly, the invention relates to a two-stroke, opposed-piston engine.
The opposed piston engine was invented by Hugo Junkers around the end of the nineteenth century. Junkers' basic configuration, shown in FIG. 1, uses two pistons P1 and P2 disposed crown-to-crown in a common cylinder C having inlet and exhaust ports I and E near bottom-dead-center of each piston, with the pistons serving as the valves for the ports. Bridges B support transit of the piston rings past the ports I and E. The engine has two crankshafts C1 and C2, one disposed at each end of the cylinder. The crankshafts, which rotate in the same direction, are linked by rods R1 and R2 to respective pistons. Wristpins W1 and W2 link the rods to the pistons. The crankshafts are geared together to control phasing of the ports and to provide engine output. Typically, a turbo-supercharger is driven from the exhaust port, and its associated compressor is used to scavenge the cylinders and leave a fresh charge of air each revolution of the engine. The advantages of Junkers' opposed piston engine over traditional two-cycle and four-cycle engines include superior scavenging, reduced parts count and increased reliability, high thermal efficiency, and high power density. In 1936, the Junkers Jumo airplane engines, the most successful diesel engines to that date, were able to achieve a power density and fuel efficiency that have not been matched by any diesel engine since. According to C. F. Taylor (The Internal-Combustion Engine in Theory and Practice: Volume II, revised edition; MIT Press, Cambridge, Mass., 1985): “The now obsolete Junkers aircraft Diesel engine still holds the record for specific output of Diesel engines in actual service (Volume I, FIG. 13-11).”
Nevertheless, Junkers' basic design contains a number of deficiencies. The engine is tall, with its height spanning the lengths of four pistons and at least the diameters of two crankshafts, one at each end of the cylinders. A long gear train with typically five gears is required to couple the outputs of the two crankshafts to an output drive. Each piston is connected to a crankshaft by a rod that extends from the interior of the piston. As a consequence the rods are massive to accommodate the high compressive forces between the pistons and crankshafts. These compressive forces, coupled with oscillatory motion of the wrist pins and piston heating, cause early failure of the wrist pins connecting the rods to the pistons. The compressive force exerted on each piston by its connecting rod at an angle to the axis of the piston produces a radially-directed force (a side force) between the piston and cylinder bore. This side force increases piston/cylinder friction, raising the piston temperature and thereby limiting the brake mean effective pressure (BMEP) achievable by the engine. One crankshaft is connected only to exhaust side pistons, and the other only to inlet side pistons. In the Jumo engine the exhaust side pistons account for up to 70% of the torque, and the exhaust side crankshaft bears the heavier torque burden. The combination of the torque imbalance, the wide separation of the crankshafts, and the length of the gear train coupling the crankshafts produces torsional resonance effects (vibration) in the gear train. A massive engine block is required to constrain the highly repulsive forces exerted by the pistons on the crankshafts during combustion, which literally try to blow the engine apart.
One proposed improvement to the basic opposed-piston engine, described in Bird's U.K. Patent 558,115, is to locate the crankshafts beside the cylinders such that their axes of rotation lie in a plane that intersects the cylinders and is normal to the axes of the cylinder bores. Such side-mounted crankshafts are closer together than in the Jumo engines, and are coupled by a shorter gear train. The pistons and crankshafts are connected by rods that extend from each piston along the sides of the cylinders, at acute angles to the sides of the cylinders, to each of the crankshafts. In this arrangement, the rods are mainly under tensile force, which removes the repulsive forces on the crankshafts and yields a substantial weight reduction because a less massive rod structure is required for a rod loaded with a mainly tensile force than for a rod under a mainly compressive load of the same magnitude. The wrist pins connecting the rods to the pistons are disposed outside of the pistons on saddles mounted to the outer skirts of the pistons. Bird's proposed engine has torsional balance brought by connecting each piston to both crankshafts. This balance, the proximity of the crankshafts, and the reduced length of the gear train produce good torsional stability. To balance dynamic engine forces, each piston is connected by one set of rods to one crankshaft and by another set of rods to the other crankshaft. This load balancing essentially eliminates the side forces that otherwise would operate between the pistons and the internal bores of the cylinders. The profile of the engine is also reduced by repositioning the crankshafts to the sides of the cylinders, and the shorter gear train requires fewer gears (four) than the Jumo engine. However, even with these improvements, a number of problems prevent Bird's proposed engine from reaching its full potential for simplification and power-to-weight ratio (“PWR”, which is measured in horsepower per pound, hp/lb).
The favorable PWR of opposed piston engines as compared with other two and four cycle engines results mainly from the simple designs of these engines which eliminate cylinder heads, valve trains, and other parts. However, reducing weight alone has only a limited ability to boost PWR because at any given weight, any increase in BMEP to increase power is confined by the limited capability of the engines to cool the pistons.
Substantial combustion chamber heat is absorbed by pistons and cylinders. In fact the crown of a piston is one of the hottest spots in a two-cycle, opposed-piston compression-ignition engine. Excessive piston heat will cause piston seizure. The piston must be cooled to mitigate this threat. In all high performance engines, the pistons are cooled principally by rings mounted to the outside surfaces of the pistons, near their crowns. The rings of a piston contact the cylinder bore and conduct heat from the piston to the cylinder, and therethrough to a coolant flowing through a cooling jacket or by cooling fins on the engine cylinder assembly. Intimate contact is required between the rings and cylinder bore to cool the piston effectively. But piston rings must be lightly loaded in two-cycle, ported engines in order to survive transit over the bridges of the cylinder ports, where very complex stresses occur. Therefore, the rings are limited in their ability to cool the pistons, which places a limit on the maximum combustion chamber temperature achievable before engine failure occurs. It is clear that, without more effective piston cooling, BMEP cannot be increased in an opposed piston engine without endangering the engine's operation.
Prior engines include an engine block in which cylinders and engine bearings are cast in a large passive unit that serves as the primary structural and architectural element of the engine. Although Bird's engine rectified torque imbalance, eliminated compressive forces on the rods, and eliminated side forces on the cylinder bore, it still used the engine block as the primary structural element, providing support for the cylinders, manifolds for cylinder ports, and cooling jackets for the cylinders and for retaining the engine bearings. But thermal and mechanical stresses transmitted through the engine block cause non-uniform distortion of the cylinders and pistons necessitating piston rings to assist in maintaining the piston/cylinder seal.